Add support for braille format files

[edwilde]

Relates to issue silverstripe/silverstripe-assets#573
and PR: silverstripe/silverstripe-framework#10970

This adds support for the Braille ASCII file format.

There is disagreement on what mime type brf files should be: datatypes.net says it should be application/octet-stream (ref), but apache says it should be text/plain (ref).

To be safe I have added both, however text/plain is likely to be the most used.

Issue

• Add support for braille format files silverstripe-assets#573

[GuySartorelli]

Thanks for this. Works great locally